1. Field of the Invention
The invention relates to a method and a circuitry for preheating and igniting a fluorescent lamp having cathodes which can be heated where, after the supply voltage has been switched on, a circuit supplied by supply voltage first preheats the lamp cathodes in such a way that no sufficient voltage for the ignition of the fluorescent lamp occurs during the preheating phase, and where an ignition voltage is applied across the fluorescent lamp after a predetermined duration of the preheating phase. The invention also relates to a device for performing this method.
2. Description of the Prior Art
High quality auxiliary circuits for running fluorescent lamps do preheat the cathodes of the lamps before applying the ignition voltage to the fluorescent lamp. This considerably extends the useful life of the fluorescent lamps when compared with an ignition in the cold state. The documents EP-A-O 118 309 and DE-OS-32 02 445 disclose circuits which provide fixed duration of the preheating of the lamp cathodes. When the duration of the preheating of the lamp cathodes is thus fixedly determined, and considering the essentially constant preheating current, different types of fluorescent lamps having different resistance values of their cathodes are being heated differently. Because the optimal temperature of the cathodes when firing a limp lies between 600.degree. and 700.degree. C., it is practically impossible to determine a single fixed value of the preheating time which would be optimal for all types of fluorescent lamps. As a consequence of the fixed preheating time and the essentially constant preheating current, it follows that one and the same ballast circuit will insufficiently heat fluorescent lamps which have cathodes with low resistance, and overheat fluorescent lamps which have cathodes with a high resistance, which results in a shortened useful life of the fluorescent lamps and in higher running costs.